The invention relates to a rotary bit for drilling a hole in a subsurface formation.
In particular, the invention relates to a rotary bit comprising a body with a central axis, a plurality of mud flow channels arranged in the outer surface of the body, said channels substantially extending from a place near the central axis to body portions having the largest radius with respect to the central axis. The bit further comprises scraping means mounted on the body and having scraping edges for drilling a hole in a subsurface formation. The scraping means extend in a single row along one side of each channel and line at least part of the wall of each channel.
Such type of bit is known from U.S. Pat. No. 3,747,699. The prior art bit comprises rows of diamonds, each row being situated along a side of a mud channel such that one plane of each diamond is flush with the wall of the relevant channel. The diamonds are effectively cooled by the mud flowing through the channels and the scrapings and flour are continuously being removed from the location where they are generated during drilling operations on the scraping edges and the planes of the diamonds that are flush with the walls of the mud channel. The cuttings and flour are removed independent of the depth to which the diamonds are scraping into the formation. Consequently, this bit can be used with equally good results in soft as well as in hard formations, since the cleaning and cooling of the diamonds will be sufficient in both cases.
The performance of the bit of the above type is excellent under normal drilling conditions. However, it has been found that when designing the bit for drilling in formations that behave plastically, problems will be encountered when such bit has to drill through an interface between hard and soft rock. On the other hand, a bit designed for drilling through interfaces between hard and soft rock without damaging the bit and/or the drill string will be found to fail in formations that behave plastically. In these latter formations, the formation material is under such stress conditions that the material is being deformed by such bit without being scraped. No formation material is then being removed from the bottom of the hole in which the bit is operating and the drilling operation is halted.
When drilling through an interface between hard and soft rock, the sudden change in hardness met by the bit when passing from the hard rock into the soft rock cannot be anticipated sufficiently quickly by lowering the weight on bit. It has been found that those bits designed for drilling in formations that behave plastically, will when drilling through said interface break up rather than drill the hard formation as soon as the thickness of the latter has decreased such that it can no longer take the high bit load. The torque exerted on the bit consequently rises shockwise and the bit will be plugged and/or get stuck and/or the drill string will be damaged severely (such as be twisted off).